1. Field of the Invention
The present invention relates to a fuel supply system for an internal combustion engine, particularly of the kind having an electronically controlled fuel injector.
2. Description of the Prior Art
As an example of a fuel supply system of the above described kind, there is known a single point injection system which has an electronically controlled single injector at a collector portion of an air induction passage or intake manifold. In a kind of single point injection system, a fuel injector is disposed upstream of a throttle valve with a view to improving the production of air-fuel mixture, as disclosed in U.S. Pat. Nos. 3,943,904 and 4,186,708 issued Mar. 16, 1976 and Feb. 15, 1980 to James C. Byrne and Lauren L. Bowler, respectively.
FIG. 3 shows a single point injection system of the above described kind. In the drawing, designated by the reference numeral 1 is a throttle body, by 2 a fuel injector disposed upstream of a throttle valve 3, by 4 an intake manifold, by 5 an electronic control unit to which signals are supplied from a throttle sensor 6, a coolant temperature sensor 7, an oxygen sensor 10, an air flow meter 11, etc., and by 12 an idle speed control valve.
With this arrangement, the quantity of fuel to be supplied into the engine per working stroke is metered by controlling the opening period of the fuel injector 2. For this purpose control pulses are supplied by the control unit 5, their duration depending on the quantity of intake air or manifold vacuum, engine speed and other correction variables. During idling, with the throttle valve 3 fully closed, the degree of opening of the idle speed control valve 12 is controlled variably depending upon variations in operation of the air conditioner, heater, lights, etc. so as to achieve a desired quantity of auxiliary air supplied through an auxiliary air passage 13.
A disadvantage of the prior art fuel supply system of the above described kind is that since the fuel injector 2 is disposed upstream of the throttle valve 3 and the auxiliary air passage 13, fuel is likely to collect in liquid form on the walls of the throttle body 1 and the auxiliary air passage 12 when an air-fuel mixture flows through the narrow space between the throttle valve 3 and the throttle body 1 and through the auxiliary air passage 13 during idling while at the same time part of fuel is likely to collect in liquid form on the upper face of the throttle valve at the time of fuel injection, thus momentarily reducing the quantity of fuel to be supplied into the engine per working stroke and therefore permitting the air-fuel mixture to become leaner than desired momentarily to result in a delay in idle speed control and therefore unstable idling.